If you go to Stanfords in Long Acre, Covent Garden, you can get a big poster map (from the A-Z company) with all the London postal areas on one side and all the London boroughs (and surrounding authorities too) on the other side.

We have one on our office wall here.

Alternatively if it's fine detail you need, the ordinary A-Z street map shows borough boundaries as a red line of dots and dashes.